X-ray scattering from the superhelix in circular DNA

Area of Science:

• Biophysics
• Structural Biology
• X-ray Scattering

Background:

• Plasmid DNA exists in various supercoiled states, influencing its biological functions.
• Understanding the precise three-dimensional structure of supercoiled DNA is crucial for molecular biology.
• Previous models proposed interwound or toroidal helical structures for superhelices.

Purpose of the Study:

• To investigate the superhelix structure of native COP608 plasmid DNA using small-angle X-ray scattering.
• To determine if the DNA adopts a toroidal or interwound conformation.
• To quantify structural parameters like pitch angle and contour length per turn.

Main Methods:

• Utilized a newly constructed position-sensitive detector for small-angle X-ray scattering (SAXS) measurements.
• Performed SAXS experiments on plasmid DNA in the presence and absence of a platinum intercalator.
• Analyzed scattering data to determine structural parameters and compare with theoretical models.

Main Results:

• SAXS patterns closely matched calculations for noninterwound helical superhelices.
• Results support a toroidal helical structure, ruling out interwound conformations.
• Quantified pitch angle and contour length per turn, estimating a specific linking difference of approximately -0.055.

Conclusions:

• Native COP608 plasmid DNA adopts a toroidal helical structure.
• Superhelicity partitioning between twist and writhe is approximately 2:1 in this toroidal model.
• The findings provide insights into DNA structural dynamics and supercoiling mechanisms.